Lesson 2 Thunderstorms BELLRINGER Quiz 8 1 If

Lesson 2 Thunderstorms

BELLRINGER: Quiz 8. 1 If a thunderstorm is not accompanied by strong, upward gusts of wind, will the thunderstorm produce hail? 8. 2 How can you tell the difference between sleet and freezing rain?

Objective l Students will know the process of a thunderstorm. l Students will gain a greater understanding of how lightning and thunder are formed.

Lesson 2: Thunderstorms l Thunderstorms are local weather phenomena that involve thunder (of course), lightning, heavy winds, driving rains, and sometimes hail. l Although you might now think so at first, they are very common. Meteorologists estimate that thousands of thunderstorms occur on the earth every day!

Updraft l All thunderstorms start in the same way. They begin with a strong updraft of air. l Updraft – A current of rising air l Usually this updraft is caused by a cold front moving in on a warm front. As you learned in the previous unit, the cold air mass tends to get under the warm air mass and lift it up, making an updraft.

l Updrafts can occur in other ways, however. l Ex. Storms can be frequent in some mountainous regions because air must rise to pass over the mountains. This can result in a storm producing updraft.

Cumulus Stage l When a strong updraft occurs, the current of air rises, causing it to cool. l As you already know, when air cools, water vapor tends to condense out of the air and form clouds. l What you may not know, however, is that when water condenses, it actually releases energy.

l This might be hard to believe at first, but look at it this way: In Unit 2, you did an experiment to show that when water evaporates, it absorbs energy, cooling whatever surface it evaporated from. l Now think about it. Water condensing is essentially the opposite of water evaporating, isn’t it?

l Well, if evaporating water tends to absorb energy and cool the surface off which it evaporated, condensation should release energy and warm the surface on which it occurs. l In a cloud, the surface is on the cloudcondensation nuclei. So condensation tends to heat up the cloud-condensation nuclei.

l This makes the updraft even stronger! l As the draft pulls more and more warm, moist air into the atmosphere, a rapidly growing cumulus cloud begins to form. l This is the cumulus stage of the thunderstorm.

The cumulus stage does not last very long. In less than 20 minutes or so, the updraft has pushed so much moist air high into the troposphere that a tall, cumulonimbus cloud has formed. These cumulonimbus clouds can get so tall that they actually reach the top of the troposphere and begin to flatten out because of the high winds blowing there. This typically gives the cumulonimbus cloud the shape of an anvil.

Mature Stage Eventually, the water droplets and/or ice crystals in the cloud become too large for the updraft to support. l At that point, one of the processes I discussed in the previous section takes over, and it begins to rain. l This marks the mature stage of the thunderstorm, which consists of heavy rain, lightning, strong winds, and sometimes hail. l

Downdrafts As the rain falls during the mature stage of a thunderstorm, it causes winds to blow downward, hitting the land spreading out in strong gusts of wind. l These downward rushes of air are called, reasonably enough, downdrafts. l These downdrafts can be quite severe if concentrated in a small enough area. l Winds of up to 170 miles an hour can be caused by local downdrafts in a thunderstorm. Sometimes they are as destructive as a tornado! l

Dissipation Stage As time goes on (typically less than 30 minutes), the downdrafts caused by the rain overpower the updrafts that started the storm, and the entire area is full of only downdrafts. l This marks the final stage of the thunderstorm, the dissipation stage. l During this stage, the rain gets lighter and lighter, and the downdrafts get less and less powerful. Eventually, the storm runs its course and the rain stops. Figure 8. 3 illustrates the stages I just described. l

Thunderstorm Cell What I just described to you is called a thunderstorm cell. l In a thunderstorm cell, there is one updraft system and one resulting thundercloud. Most thunderstorms consist of several such cells. l Thus, although a single thunderstorm cell usually lasts for much less than an hour, a thunderstorm composed of several cells can last a lot longer. l

l Thunderstorms, of course, are best known for their thunder and lightning. These two effects always accompany one another. I am sure you have wondered where thunder and lightning come from. l Perform the following experiment to find out.

BELLRINGER: QUIZ 8. 3 If a thunderstorm produces hail, in which stage of the thunderstorm would it come? 8. 4 A thunderstorm begins, raining heavy sheets of rain for more than an hour before the rain begins to lighten. Was this thunderstorm composed of one cell or many cells?

What happened in the Experiment? In your experiment, you should have see a bluish -purple spark leap from the balloon to your hand. Believe it or not, that was a small lightning bolt. l Obviously it did not have the power of the lightning that accompanies a thunderstorm, but it was produced in a similar way. l The snap that you heard when the spark formed was actually thunder; it just didn’t have the power of thunder in a storm. l

How did the lightning form in your experiment? When you rubbed the balloon against your hair, the balloon picked up some negative electrical charges from your hair. l Remember from Unit 4 that all matter (including your hair, the balloon, and your fist) contains positive and negative electrical charges. l When the balloon picked up some of the negative charges in your hair, it became negatively charged. Since your hair lost those electrical charges, it became positively charged. l This explains why your hair tends to stand up when you rub a balloon in it. Your positively charged hair is attracted to the negatively charged balloon, and it stands up to get closer to the balloon. l

Once you got the balloon to be negatively charged, you passed it near your fist, which has just as many positive charges as negative charges in it. l When the negatively charged balloon came close to your fist, the positive charges in your fist were attracted to it. Thus, they moved in your fist as to be as close to the negatively charged balloon as possible. l When they reached you skin, however, they had to stop because electrical charges do not move well in air. l

Because of this, air is called an insulator. Insulator – A substance that does not conduct electricity very well. l Eventually, the electrical charge buildup became so strong that the air was forced to allow some of the positive charges to move toward the balloon. l Even though the electrical charges began to move in the air, they had a hard time traveling through it because air is an insulator. l Thus, they lost a lot of energy. l l

l l l That energy was transformed mostly into light. The light, obviously, was the spark that you saw. The rest of the energy heated the surrounding air. The heat was rather substantial, and it happened very quickly. As a result, the hot air began rushing outward, forming waves of air. In Unit 6, what did you learn about sound?

l Sound moves as a wave through air and other substances. l Thus, the motion of the hot air actually forms sound waves. Those sound waves created the crackle you heard.

Most likely, you didn’t see the lightning in step #7 of the experiment. Why not? l Well, remember that in order for the changes to travel from your hand to the balloon, they need to build up enough to overcome the insulating properties of the air. l When you had a fist formed, the positive charges moved as close as they could to the balloon, so they built up in your knuckles, because your knuckles were closer to the balloon than was the rest of your fist. l

Since this concentrated the positive charge, the charge was able to overcome the air’s insulating properties, making the spark. l When you had your hand flat, the charge did not concentrate anywhere. Instead, it spread out along your hand. l Thus, your probably felt the electrical buildup over a larger area, but you never saw a spark because it was not concentrated enough at any one spot to overcome the air’s insulating properties. l

l Lightning forms in almost an identical way as did the spark in your experiment. l Take a look at Figure 8. 5

How Lightning Forms l In a large cumulonimbus cloud, the heavy ice crystals near the top of the cloud eventually are too heavy to be supported by the updraft that formed the cloud in the first place. l The ice crystals therefore begin to fall through the cloud, starting the Bergeron process we talked about in the first lesson.

Since a thundercloud is so large, the ice crystal makes several collisions with water droplets and other ice crystals on the way down. l When those collisions are glancing collisions rather than head-on collisions, electrical charges can be transferred, much like what happened when the balloon rubbed up against your hair in the experiment. l

Stepped Leader This transfer of charge that occurs in glancing collisions results in an imbalance of electricity within the cloud. l The bottom of the cloud starts building up negative charge (like the surface of the balloon in the experiment), and the top of the cloud starts building up positive electrical charge (like your hair in the experiment). l The negative charges at the bottom of the cloud attract the positive charges in the ground (like the positive charges in your hand were attracted to the balloon). l This causes a buildup of positive electrical charge on the ground. This positive electrical charge causes some of the negative charge in the cloud to move in a jerky, stepwise fashion toward the earth. This jerky movement of negative charges toward the earth is called a stepped leader. l

Return Stroke Since the negative charges are moving through air, they lose a lot of energy due to the insulating properties in the air. l Thus, a stepped leader is often accompanied by a dim spark. l As the negative charges get closer to the ground, however, the positive charges build up more force, eventually overcoming the insulating properties of the air and rushing up to meet the negative charges that are moving down. l This return stroke, as it is called, is responsible for most of the light and sound of a lightning strike. l

Notice where the lightning bolt strikes in the figure. l It strikes the tree. Just like the spark in the experiment tended to hit a knuckle because it was closer to the balloon, lightning tends to hit things that are tall. l That’s because the positive charges concentrate on those things, trying to get as close to the negatively charged cloud as possible. l That’s why it is dangerous to stand under trees during a thunderstorm. The trees may shield you from rain, but they tend to attract lightning bolts! l

Thunder Just as the spark in the experiment caused a snapping sound, the return stroke in lightning causes the booming sound we call thunder. l This is caused by the intense heating that results when the charges push their way through the air. l The charges lose so much energy fighting their way through air that a single lightning bolt can heat the surrounding air to temperatures in excess of 50, 000 °F! l This superheated air quickly rushes outwards in waves, forming sound waves that our ears detect as thunder. l

l The lightning I just described to you is called cloud-to-ground lightning. l Although this is what we usually think of when we hear the work “lightning, ” it is not the most common type of lightning that occurs in thunderstorms.

Cloud-to-Cloud Lightning The typical lightning that accompanies thunderstorms is called cloud-to-cloud lightning and is commonly referred to as “sheet lightning, ” l This kind of lightning is caused by the same process, but the electrical building and charge transfer occurs between two clouds, not between the ground a cloud. l This kind of lightning lights up the sky in big sheets instead of striking the ground in a bolt. l Since the air is still heated as a result of the charges moving, however, there is still thunder. l